Electrochemical cell

ABSTRACT

An electrochemical cell in the form of first and second sheet members ( 2,12 ) at least one of which is gas permeable on which is disposed one or more planar electrodes ( 4,6,8 ). Peripheral regions of the first and second sheet members ( 2,12 ) are sealed together to form a sealed envelope or reservoir containing electrolyte. Electrical connection means ( 41,61,81 ) extend from each of the electrodes ( 4,6,8 ) across the sealing of the sheet members ( 2,12 ) to provide external electrical connection.

This application is a 371 of PCT/GB97/03372 filed Dec. 5, 1997.

FIELD OF THE INVENTION

This invention relates to electrochemical cells and in particularalthough, not exclusively to electrochemical cells for use in gassensors and fuel cells.

BACKGROUND ART

An electrochemical gas sensor for sensing an oxidisible or reducible gas(e.g. carbon monoxide) in the atmosphere usually contains a sensing orworking electrode, a counter electrode and an inlet (usually a diffusionbarrier) to allow the atmosphere to permeate to the sensing electrode.Both electrodes are in contact with an electrolyte in order firstly toproduce an electrochemical reaction at the sensing electrode with thegas to be sensed, and secondly to produce an electrochemical reaction atthe counter electrode with oxygen in the atmosphere, electrolyte orother gas source. Current is carried through the electrolyte by ionsproduced in the reaction and by electrons through an external circuit,the current in the circuit indicating the gas concentration. A referenceelectrode may be employed in combination with a potentiostat circuit tomaintain the potential between the sensing electrode and the cellelectrolyte in order to increase stability of operation.

In terms of physical construction, the sensor normally comprises anexternal housing which acts as a reservoir for the electrolyte, a wickor matrix to hold the electrolyte in contact with the electrodes, andexternal electrical terminals making electrical connection with theelectrodes. The majority of present sensor cells use a stacked electrodearrangement, as for example in U.S. Pat. No. 4,406,770.

There has been recent proposals for design simplification, see forexample U.S. Pat. No. 5,183,550 which discloses a gas sensor in whichthe sensing, counter and reference electrodes are mounted in a commonplane on a common ceramic substrate, with contact leads extending fromthe electrodes to the other surface of the substrate for electricalconnection.

Our copending application WO 96/14576 (our ref. PQ12622) discloses andclaims a gas sensor comprising a substrate, electrodes formed as porousplanar elements on the substrate, and the substrate being porous topermit permeation of gas to the electrodes from the environment, ahousing containing a reservoir of electrolyte and external terminalsmounted to the housing, wherein the substrate is bonded to the housingby the application of pressure and heat, so that in a single assemblyoperation, the housing is sealed and electrical connections are made tothe electrodes while blocking the porosity of the electrodes to preventelectrolyte permeating through the electrode to the area of theelectrical connection.

While the above construction represents a considerable advance in termsof cost reduction, nevertheless inexorable demands for cost reductionwithout sacrificing quality, forces further design simplifications.

SUMMARY OF THE INVENTION

This invention is based on a concept of providing two flexible plasticssheets, one having electrode areas printed thereon, which are bondedtogether around their edges to form a reservoir for electrolyte, ratherin the form of a tea bag.

Accordingly, the present invention provides in a first aspect, anelectrochemical cell comprising at least first and second sheet members,at least one of the sheet members including a gas permeable section onwhich is disposed one or more planar electrodes, peripheral regions ofthe first and second sheet members being sealed together to form areservoir containing electrolyte, and including electrical connectionmeans extending from each of said electrodes across the sealing of thesheet members for external electrical connection.

In a further aspect, the invention provides an electrochemical cellassembly formed of a plurality of sections, each section having firstand second, sheet members, each sheet member including a gas permeableregion on which is disposed a planar electrode, peripheral regions ofthe first and second sheet members being sealed together to form areservoir containing electrolyte, and including electrical connectionmeans extending from said electrodes across the sealing of the sheetmembers for external electrical connection, said assembly includingmanifold means for directing first and second gases to each section soas to contact respective first and second sheet members.

In the latter embodiment of the invention, by forming the electrodes ondifferent sheet members, it is possible to arrange a first gas to flowover the first sheet member, and a second gas over the second sheetmember. For example with a suitable manifold structure, the assembly mayconstitute a fuel cell.

The construction of the present invention, either as a gas sensor orfuel cell, is extremely simple and permits substantial cost savings.

Various specific forms of construction are possible. Both sheet membersmay be flexible, the flexibility permitting the insertion of electrolytebetween them. Both sheet members may be part of a single sheet, which isfolded over so that the sheet members are face to face. Alternativelyone or both sheet members may be formed from a sheet which is performedto have a three dimensional shape, for example one sheet may have a wellformed therein to define a reservoir space. Alternatively in onepreferred construction, one sheet may be formed as a planar sheet ofporous PTFE carrying the required configuration of electrodes, and aflexible plastics closure sheet may be welded to the edges of the PTFEsheet to define the reservoir.

A third sheet member may also be employed, for example an intermediatelayer of lower melting point for sealing the two outer sheets. The thirdsheet may be of highly porous material to hold the electrolyte. In afurther form, the third sheet member may define a second reservoir spacefor a second electrochemical cell.

The invention thus permits very thin assemblies (of the order of 1 to 2mm) to be produced in a variety of shapes. This permits applications fora gas sensor where space is extremely limited, e.g. on or in a person'sclothing. The extreme cheapness of production provides the possibilityof “disposable” sensors which may be used only once or a small number oftimes and then disposed of.

Thus production and assembly may be simplified by printing theelectrodes on a sheet of flexible substrate material which then isfolded over one or more times, or placed against another sheet ofmaterial and then sealed around the edges to form a “tea bag” typestructure. The wick and electrolyte are contained within the bag and thesealing may be by heat, adhesive or mechanical force. Such devices maybe made in irregular shapes, and are suitable for high levels ofautomation for cost reduction.

Where sensors of a completely flexible construction are produced, theywill in practice normally be attached to a rigid support or mounted in arigid housing to prevent spurious noise due to bending. Due to thecompact nature of the thin sensor, it may be disposed with controlelectronics on to a single substrate. This substrate may also have thediffusion-limiting gas access built in.

The electrochemical cells in accordance with the invention lendthemselves to automatic fabrication and assembly on a production line.Thus sheet material stored in one or more rolls can be unwound andsuperimposed, and a pattern of cells then pressed, cut and sealed insimultaneous operations from the sheets. For a fuel cell, the cells maybe formed as flat arrays, e.g. 4×4 on the sheets.

While the electrolyte in the cells is usually in the form of a liquid,it may be in the form of a gel or solid polymer, pasted or otherwiseaffixed to the electrodes.

For an electrode printed on a gas permeable membrane three functionshave to be achieved to assemble it into an electrochemical cell: 1) tomechanically attach the electrode, 2) seal in the electrolyte and 3) toprovide a conductive path from the electrode to the outside of the cell.

A preferred construction method, similar to that as described in WO96/14576 uses the technique of heat sealing a porous PTFE sheet member(the electrode) to the cell body component As the printed electrode runsthrough this seal all three needs are met with the addition of no extracomponents and in addition the ability to automate is provided.

The sheet members may also be glued together; this method can achieveall three requirements for cell assembly. In practice it makes use ofthe porosity of a PTFE membrane to achieve adhesion. This method ofconstruction is important to allow the assembly of fragile or complexelectrodes. Fragile electrodes may result from cost reduction, forexample. Complex electrodes include irregular shapes, multiple prints orsealing in more than one plane e.g. around the outside of a moulding.This assembly method may be important for small disposable sensors(limited life). Advantages: very little disruption of the electrode inkstructure, the adhesive can enter the porosity of the electrode ink andsubstrate increasing strength, and the method is very adaptable. Withthis method, different methods of electrical connection to theelectrodes may be employed, for example electrode lead wires extendingfrom the electrodes to external electrical connections.

The ability to produce multiple cell assemblies at low cost is ideal forfuel cell fabrication. The production of cells in a strip form may beadapted to give an array (say 3×4), this array forming a single layer ina stacked assembly. One electrode of each cell is on the top of thelayer and the other underneath, to allow the air and fuel gases to besupplied via simple manifold systems to the relevant electrode.

A preferred stacked assembly includes rigid spacer devices interleavedwith the layers of cells and providing a manifold structure on each sideof each layer to permit inflow of an appropriate gas. First manifolds onone side of the layer are disposed to allow inflow of a fist gas fromone direction and second manifolds are disposed on the other side of thelayer are arranged at right angles to the first manifolds to allowinflow of a second gas from a direction at right angles to the first Byadjusting the size and shape of the manifolds the volume or flow rate ofthe two gases could be set as desired.

In each layer the cells may be connected in series to dictate thevoltage generated by the cell, and the layers may be connected togetherin parallel in order to define the maximum current of the assembly.

A second stack assembly is to have all the layers fabricated in one longstrip which is folded up in a “zigzag” pattern. A rigid manifold/spacerarrangement is employed to permit air flow through the stack at rightangles to the fold pattern.

In the case of a fuel cell, it is possible that the sheet members maynot be completely sealed around their peripheries, but that a smallunsealed region may be provided in order to provide a vent for water orother liquid generated during operation within the cell.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will now be described withreference to the accompanying drawings wherein:

FIG. 1 is a sectional view of an embodiment of an electrochemical cellfor use as a gas sensor in accordance with the invention;

FIG. 2 is a plan view of the sensor of FIG. 1;

FIG. 3 is a sectional exploded view of the gas sensor of FIGS. 1 and 2incorporated in a gas sensor for personal use;

FIG. 4 is a sectional view of an embodiment of an electrochemical cellfor use as a fuel cell element in accordance with the invention; and

FIG. 5 is a perspective view of part of the fuel cell element of FIG. 4,showing the how the element is mounted in a fuel cell stack of suchelements.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 1 and 2, there is shown an electrochemical cellin the form of a gas sensor comprising a flexible porous PITE substrate2, 0.25 mm thick onto which are printed a reference electrode 4, aworking electrode 6, and a counter electrode 8. As shown in FIG. 2,these electrodes are generally rectangular and are disposed side by sidewith the working electrode 6 disposed in the centre so that thereference and counter electrodes can perform their functions withoutinterfering one with the other. Portions of the printed electrodes 41,61, 81 extend to one edge of substrate 2 for making external electricalconnection.

The electrodes are deposited onto the substrate 2 by, for example,screen printing, suction or vacuum depositing selected areas from asuspension placed onto the substrate, spray coating, or any other methodsuitable for producing a patterned deposition of solid material.Deposition may be of a single material or of more than one materialsequentially in layers, so as, for example, to vary the properties ofthe electrode material through its thickness or to add a second layer ofincreased electrical conductivity above or below the layer which is themain site of gas reaction.

The electrodes are printed on to a gas impermeable substrate 2 which isself supporting. This has the advantage of making the electrode stifferand so less susceptible to vibration and shock and makes it moredifficult to rupture or pierce the electrode.

A wick 10 of a highly absorbent material and containing a charge ofelectrolyte liquid or gel is positioned over the electrodes, and aflexible plastics sheet 12 of polypropylene, 0.1 mm thick, having alower melting point than PTFE, is positioned over the wick and substrateto define a reservoir space 14 for the electrolyte. Sheet 12 has thesame shape and size as that of sheet 2 in plan, and the edges of thesheets are bonded together in a rectangular bond line 16 by anappropriate pressing tool which applies suitable heat and pressure tothe to effect the bond. In the region where the bond extends over theconnecting regions 41, 61, 81, the sealing process causes the porosityof the electrode material to be blocked, which forms a barrier to theegress of electrolyte from the wick 10. The resultant sensor has athickness which is not greater than 2 mm.

In use, gas to be sensed permeates through the PTFE substrate 2 to theworking electrode 6. The rate of diffusion of the gas to be sensed iscontrolled by the porosity of the substrate 2 so the substrate becomesthe diffuser control for the cell.

Referring now to FIG. 3, this shows an electrochemical cell suitable foruse as a gas sensor designed for personal use in the form of a cardwhich may be placed in the pocket of a person's clothing or attached tothe exterior of the clothing. The card comprises a stiff plasticssubstrate 20 on which is mounted a electrochemical cell 22, of the formshown in FIGS. 1 and 2. An electronics chip 26 is also mounted onsubstrate 20 and is electrically connected to the cell 22. Theelectronics chip 26 is manufactured by a CMOS process and provides apower supply (a battery or solar powered cell is provided), controlcircuitry for energising the electrodes of the cell and output means toprovide an alarm signal. A cover 28 is provided for covering the cell 22and chip 26. An aperture 30 is provided in the substrate fordiffusion—limited gas access to the PTFE substrate 2 of the cell 22.

Referring now to FIGS. 4 and 5, there is shown an electrochemical cell40 for use in a fuel cell constructed in accordance with the presentinvention. The cell 40 comprises first and second flexible sheets 42,44, each being formed of gas permeable PTFE and on each inside surfaceof which is printed a respective electrode area 46, 48. A wick 50containing a charge of electrolyte material is disposed between theelectrodes. The two sheets 42, 44 are rectangular in configuration andare bonded to one another along their four edges to form a sealedenvelope, or reservoir is for the electrolyte. Although not shown in thedrawings, portions of the electrode areas extend through the bondedregion to the edges of the sheets for external electrical connection ina similar way to that shown in FIGS. 1 and 2. First and secondrectangular gas manifolds 52, 54 are placed above and below the fuelcell element 40 with flanges 56 of side walls 58 clamping the cell 40between the manifolds. The manifolds permit gas access of first andsecond gases; one gas usually being fuel and the other air respectivelyto the each side of the cell 40. The fuel and aim diffuse through therespective sheets 42, 44 and react with the respective electrode 46, 48.Thus electrochemical reactions are created at the electrodes 46, 48,generating an electrical output at the external electrical connections.

As indicated in FIG. 5, a multiplicity of cells 40 are stacked in avertical stack 60, a first gas entering the stack from one side 62 ofthe stack at right angles to a side 64 at which a second gas enters.External connections to the cells 40 are interconnected so as to providea combined electrical output from the stack of cells 40.

What is claimed is:
 1. A gas sensor consisting of an electrochemicalcell, comprising: a first sheet member having thereon a gas permeablesection, said gas permeable section having disposed thereon a pluralityof planar electrodes; a second sheet member, wherein peripheral regionsof said first sheet member and said second sheet member being sealedtogether to form a reservoir therebetween, said reservoir being adaptedto contain electrolyte; and electrical connection means extending fromeach of said plurality of planar electrodes to said peripheral regionsof said first sheet member and said second sheet member, said electricalconnection means being adapted to provide a respective externalelectrical connections for said plurality of planar electrodes.
 2. A gassensor consisting of an electrochemical cell according to claim 1,further comprising: a third sheet member disposed between said firstsheet member and said second sheet member.
 3. A gas sensor consisting ofan electrochemical cell according to claim 1, further comprising: athird sheet member having peripheral regions thereof being sealed tosaid first sheet member and said second sheet member to define a secondreservoir.
 4. A gas sensor consisting of an electrochemical cellaccording to claim 1, wherein: said first sheet member comprises: a flatporous PTFE sheet having said plurality of planar electrodes formedthereon, and wherein said second sheet member comprises: a flexibleplastic sheet.
 5. A gas sensor consisting of an electrochemical cellaccording to claim 1, wherein: said plurality of planar electrodescomprises: a first electrode; a second electrode; and a third electrode,wherein said first electrode, said second electrode and said thirdelectrode are disposed side by side on said first sheet member.
 6. A gassensor consisting of an electrochemical cell according to claim 1,further comprising: a stiff planar substrate member having mountedthereon said reservoir formed by said first sheet member and said secondsheet member, and a electrical circuit means adapted to operate saidelectrochemical cell as a gas sensor.
 7. A gas sensor consisting of anelectrochemical cell according to claim 1, wherein: said peripheralregions of said first sheet member and said second sheet member aresealed together by application of pressure and heat.
 8. A gas sensorconsisting of an electrochemical cell according to claim 1, wherein:each of said plurality of planar electrodes has a portion extendingthrough said sealed together peripheral regions of said first sheetmember and said second sheet member to define said electrical connectionmeans.
 9. A gas sensor consisting of an electrochemical cell accordingto claim 1, wherein: said electrolyte is a liquid electrolyte, and isheld in a porous member.
 10. A gas sensor consisting of anelectrochemical cell according to claim 1, wherein: said electrolyte isa gel electrolyte, and is affixed to surfaces of said plurality ofplanar electrodes.
 11. A gas sensor consisting of an electrochemicalcell according to claim 1, wherein: said electrolyte is a polymerelectrolyte, and is affixed to surfaces of said plurality of planarelectrodes.
 12. A gas sensor consisting of an electrochemical cellassembly, comprising: a plurality of sections; and manifold means,wherein each of said plurality of sections comprises: a first sheetmember; and a second sheet member, wherein each of said first sheetmember and said second sheet member has a gas permeable region on whichat least one planar electrode is disposed, peripheral regions of saidfirst sheet member and said second sheet member being sealed together toform a reservoir therebetween adapted to contain electrolyte, whereineach of said plurality of sections has electrical connection meansextending from said at least one planar electrode across said sealedtogether peripheral regions of said first sheet member and said secondsheet member, said electrical connection means providing externalelectrical connection, and wherein said manifold means directs a firstgas and a second gas to each of said plurality of sections, said firstgas being directed to contact one of said first sheet member ofrespective one of said plurality of sections, and said second gas beingdirected to contact said second sheet member of said respective one ofsaid plurality of sections.
 13. A gas sensor consisting of anelectrochemical cell assembly according to claim 12, wherein: each ofsaid first sheet member and said second sheet member comprises: a PTFEsheet.
 14. A gas sensor consisting of an electrochemical cell assemblyaccording to claim 12, wherein: said plurality of sections are arrangedin a rectangular matrix to define a plurality of fuel cell elements. 15.A gas sensor consisting of an electrochemical cell assembly according toclaim 12, wherein: said plurality of sections are arranged in a singlerow to define a plurality of fuel cell elements.
 16. A gas sensorconsisting of an electrochemical cell assembly according to claim 14,wherein: each of said plurality of sections are stacked on top of eachother.
 17. A gas sensor consisting of an electrochemical cell assemblyaccording to claim 15, wherein: said plurality of sections arranged in asingle row is folded to place at least one of said plurality of fuelcell elements on top of another one of said plurality of fuel cellelements.
 18. A gas sensor consisting of an electrochemical cellassembly according to claim 12, wherein: each said plurality of sectionscomprises: a matrix of fuel cells, and wherein each of said plurality ofsections are stacked on top of each other.
 19. A gas sensor consistingof an electrochemical cell assembly according to claim 12, wherein: eachsaid plurality of sections comprises: rigid spacer means for definingsaid manifold means for each of said plurality of sections.
 20. A gassensor consisting of an electrochemical cell assembly according to claim12, wherein: said manifold means is arranged to direct said first gasand said second gas from opposite sides of said first sheet member andsaid second sheet member, respectively.
 21. A gas sensor consisting ofan electrochemical cell assembly according to claim 12, wherein: saidperipheral regions of said first sheet member and said second sheetmember are sealed together by applying pressure and heat.
 22. A gassensor consisting of an electrochemical cell assembly according to claim12, wherein: said at least one planar electrodes has a portion extendingthrough said sealed together peripheral regions of said first sheetmember and said second sheet member to define said electrical connectionmeans.
 23. A gas sensor consisting of an electrochemical cell assemblyaccording to claim 12, wherein: said electrolyte is a liquidelectrolyte, and is held in a porous member.
 24. A gas sensor consistingof an electrochemical cell assembly according to claim 12, wherein: saidelectrolyte is a gel electrolyte, and is affixed to surfaces of saidplurality of planar electrodes.
 25. A gas sensor consisting of anelectrochemical cell assembly according to claim 12, wherein: saidelectrolyte is a polymer electrolyte, and is affixed to surfaces of saidplurality of planar electrodes.